In a mobile communication system, an area where a service is provided is divided into plural cells, and a base station is placed in each of the plural cells. When a mobile station moves across cells, the mobile station switches a base station that is a communication counterpart to another base station. When a mobile station switches to a base station with which it communicates to another base station, this is called a “handover”. There are different types of handover: an “intra-frequency handover”, or a “soft handover”, in which communication frequencies before and after switching are the same, and an “inter-frequency handover” in which communication frequencies before and after switching are different.
The inter-frequency handover has a higher probability of being subject to a call disconnection and requires greater power consumption than the intra-frequency handover.
The inter-frequency handover includes: (1) a method in which, after a network determines that the inter-frequency handover should be implemented and transmits an inter-frequency handover execution notification to a mobile station, the inter-frequency handover is implemented at the mobile station and at the network; and (2) a method in which, after a mobile station determines that the inter-frequency handover should be implemented and transmits an inter-frequency handover execution notification to a network, the inter-frequency handover is implemented at the mobile station and at the network. In either method, the implementation of the inter-frequency handover is determined based on a measurement result of a transmission power at a base station or a result of a measurement (hereinafter referred to also as “inter-frequency measurement”) of a radio communication quality of an inter-frequency cell (different-frequency cell) measured at a mobile station. For example, we assume a case in which a mobile station communicates with a base station at a frequency f1 and the mobile station communicates with a next base station at a frequency f2. For example, the inter-frequency handover can be implemented in conditions in which the difference between the radio communication quality at frequency f1 and the radio communication quality at frequency f2 exceeds a threshold.
Patent Document 1 describes, as a control method for such an inter-frequency handover, a technique for implementing an inter-frequency handover, having considered a moving speed of a mobile station. Specifically, a moving speed of a mobile station is measured, and a threshold is changed depending on the moving speed. According to this technique, the threshold is made higher as the moving speed becomes greater, such that the number of times of implementing the inter-frequency handover can be reduced in a case in which the moving speed is high.